In the production of fabrics like denim it is necessary to dye the entire warp, that is all of the parallel filaments that will eventually be woven as the warp into the fabric, a relatively dark color. For blue denim at a vat dye like indigo is used which is applied in several different stages to the filaments being dyed, alternating with drying or fixing stages during which the dye oxidizes to the desired blue color. Many different types of vat dyes require such multiple application alternating with a drying or fixing stage.
The classic system for carrying out this procedure, (see "Kontinuefarben von Baumwollkettgarn mit Indigo" by P. Richter in Textileveredlung 10 (1975) PP 313-317) takes all of the filaments used eventually to form the warp as bundles or cables each having 300-400 filaments. Each such cable or ball warp is up to 15,000 m long. A plurality of such cables, normally no more than twenty-four, are passed through a bath at between 30.degree. C.-50.degree. C. to wet them, and then the liquid is squeezed out of them and they are rinsed in cold water and again squeezed dry. Subsequently the ball warps are passed successively through between four and six different dye baths. On emerging from each of the dye baths the cable or strand is squeezed as dry as possible and is then subjected to a drying or gas-treatment stage.
The synthetic indigo dye is made water soluble in a chemically reducing bath. After having most of the dye squeezed out of the textile the textile dries with simultaneous oxidation of the dye which changes to the desired blue color and simultaneously becomes insoluble in water. In order to achieve the desired darkness it is necessary, as mentioned above, to repeat the dyeing and gas-treatment stages between four and six times. Thus such a standard dyeing system requires between four and six separate baths, each provided with its own pair of squeeze rollers and each having a separate gas-treatment rack.
After such multiple dyeing and drying the strand is washed, rinsed, brightened if necessary, and dried. The cables must then be painstakingly taken apart and the warp filaments rebeamed, an operation which is extremely laborious and time-consuming. Subsequently the rebeamed filaments are normally fed to a sizing machine, whereupon they can be employed as a warp beam on a weaving operation.
As a result of the thickness of the cable that is dyed and the inherent differences between the dye concentrations in the various baths it is obvious that the hue is going to vary somewhat from filament to filament and along each filament. Nonetheless the variations normally lie within a certain relatively narrow range so that when the filaments are rebeamed the color equalizes out over the fabric eventually produced. Indeed the slight variation often gives what is considered a desirable effect.
Obviously the disadvantage of this system is that the amount of equipment necessary for dyeing is extremely large. Six separate dyeing vats, each containing over 1000 liters of dye, must be provided, each with a respective pair of pinch rollers normally driven by a respective 5 kW motor. In addition each vat is associated with a separte drying rack comprised of a plurality of vertically offset rollers that guide the filaments through a vertically sinusoidal path, with the filaments engaging each roller over approximately 180.degree..
Operation of the system is relatively complex. First of all, the filaments of a given cable must all be under approximately the same tension. It is difficult to produce this simply by providing threadbrakes at the feed location, as that filament on the creel furthest from the takeup location will normally be tensioned substantially differently from that of the closest portion on the creel. If the tension is uneven a filament will break, normally winding itself about one of the guide rollers so that when that portion of the bundle about which is provided a temporary holding thread arrives at this roller the holding thread will normally be broken and at times the entire bundle ruined. Thus it is necessary for the operator of the machine to pay extremely close attention to its operation in order to shut it down at any time if a thread breaks and starts to wind around one of the guide rollers. When such an accident occurs the operation must be shut down, normally holding a portion of the cable under the dye too long and ruining at least one batch. Repair entails painstakingly threading the cable back through the extremely lengthy path it must follow in the machine.
A newer system is that of so-called sheet-dyeing. Here the filaments are all kept in a planar array, one next to the other, just as they would be used on the eventual warp beam. This type of strand is then passed in the same manner as the above-described cable through a plurality of vats, normally between four and six, each again provided with a respective pair of pinch rollers and drying rack. The advantage of this system is that the filaments remain in the position they will be in in the warp beam, so that the painstaking undoing of the cables and rebeaming of the filaments is avoided. Nonetheless this system has a considerable disadvantage that the dye hue is normally quite irregular in the finished product. This irregularity is normally manifested as longitudinal warp-wise stripes of lighter and darker colors in the dyed warp beam. Such strips, if at all prominent, create an extremely undesirable effect in the finished goods, normally making them unacceptable for high quality use.
The main cause of this irregularity has been traced to the inability of the pinch rollers to squeeze most of the dye uniformly out of the array of parallel filaments as they emerge from the dye. The layer is simply too thin for effective operation of the pinch rollers, one of which is normally a hardened steel cylinder and the other a hard rubber cylinder urged against it with several tons of force. Thus more dye is left on some filaments than on others, with the eventual above-discussed stripe effect.
A further development of this sheet-dyeing procedure has been to dye several such separate warps at the same time. This procedure increases the thickness of the multiple strand which passes through each of the set of pinch rollers, so that they can effectively reduce the liquid content thereof uniformly. Furthermore simultaneously dyeing two different warp beams at the same time substantially increases the output of a single dyeing installation, especially when the fact is taken into account that the painstaking formation of cables and rebeaming according to the older system is eliminated.
Even with this relatively efficient last-discussed system it is normally not possible to pass the filaments in the same production operation through a sizing machine. The main reason is that filament breakages are inevitable, and such filament breakages require that the machine be shut down at least temporarily. A warp beam cannot be held stationary in a standard sizing machine, so that it is normally necessary to wind each of the warp beams up on an individual beam, and then pass them separately through a sizing machine.
Furthermore this improved method requires that at least four and normally six baths each containing 1000 liters of dye be used. Each of these baths is consumed and must periodically have added to it, in the case of synthetic indigo, more dye, the chemicals which added with caustic soda reduce the dye to make it soluble, normally hydrosulphite, and the necessary surface active agents insuring proper penetration of the dye into the textile and further dispersing agents to maintain the suspension. Furthermore it is necessary to maintain the baths at a cooler temperature, normally below 20.degree. C., as above this temperature the previously fixed dyes would be rereduced and dissolved. Furthermore temperature variations, like variations in chemical makeup of the bath, produce variations in hue. The classic variation is one from the head to the foot of the strand being dyed, normally a lightening hue from the head to the foot as the dye baths weaken and the temperatures increase.
Thus creating a uniform hue in a given batch is a relatively difficult operation requiring constant monitoring of bath composition, cooling of the bath composition, and removing of any foreign material carried by the strand into the baths. This problem normally requires that the textile be painstakingly washed before dyeing. What is more the dye exposed at the surface of the bath frequently oxidizes all by itself, creating another problem in weakening of the bath.
Thus it is normally possible only to use a speed of approximately 20 meters per minute through the bath if good dyeing is to be achieved. What is more the gas treatment, which is normally a simple drying although it can entail an active heating, radiation with ultraviolet or infrared light, or other operation in the air, must be uniform from vat to vat, that is the temperature and treatment at each of the fixing or drying racks must be identical. Simply put, operating such a system is extremely difficult, entailing keeping track of and controlling a great many variables all within a relatively narrow range. If it becomes necessary to change the hue substantially it is normally impossible to do so simply by redosing any of the baths. Instead all of the baths are normally dumped out and new batches of dye are made up. The amount of chemicals involved is extremely large, so that such discarding of four to six vats full of dye represents a considerable waste. Furthermore it is normally impossible to keep these dye baths for long periods of time, so that in the event of a holiday shutdown or the like all of the baths must be drained out and replaced at the end of the break.
Another disadvantage of the known system is that the considerable amount of liquid entailed creates a considerable pollution problem. The offensive chemicals, such as the caustic soda, in the dye vats cannot simply be discharged into a local sewer system. Instead complex treatment apparatus must be provided for the 4000 liters-6000 liters of dye liquid in each batch.
Another problem with the known system is that each of the drying racks normally has twelve deflecting rollers, creating 30 meters of path in the gas-treatment zone at the drying rack for each vat. Each of these rollers is engaged by the strand over about 180.degree., so that the contact between the strand and the great number of rollers is considerable. The likelihood of a filament breaking and winding up on one of these rollers is increased with the number of rollers and the amount of contact angle. Obviously with such a large system the possibility of such breakage and winding-up is great.
What is more the overall length of such a system is normally at least 40 meters. Each of the set of pinch rollers is operated, as mentioned above, by a respective 5 kW motor. The power consumption for such a large and complex system is therefore also relatively great.